Rotation of the outer disc from classical cepheids.

Radial velocities and distances have been measured for a sample of 48 remote classical cepheids located in the outer disc of the Galaxy (118deg<l<274deg). The distances are determined from BVI photometry, with semi-empirical metallicity corrections calibrated on the Magellanic Clouds. Using these cepheids as tracers, the rotation curve of the disc is determined between R_0_ and 2R_0_. The result is a flat rotation curve about 30km/s lower than theta_0_, V_rot_=193+/-4km/s for R_0_==8.5kpc and theta_0_=220kms/s assumed, or V_rot_=167+/-4km/s for R_0_=8kpc and theta_0_=200km/s. The possible presence of non-axisymmetric components in the rotation of the outer disc is considered. We find a very small or vanishing value for any radial motion of the LSR or expansion/contraction motion. In particular, the data exclude large radial motions such as have been proposed in some models to account for features observed in the kinematics of the gas. The rotation curve indicated by cepheids is markedly lower than that currently derived using HII regions. Possible explanations are examined. A real kinematical difference is possible, caused for instance by radial motions in the gas induced by spiral arms, but an important distance scale difference is not excluded. A systematic error added to a high dispersion on HII region distances could explain the mismatch.